1. Field Of The Invention
The present invention relates generally to downhole tools, and particularly relates to tools utilizing compressible fluid to help restore an actuating piston to an original position thereof.
2. Description Of The Prior Art
It is well known in the art that downhole tools such as testing valves, circulating valves and samplers can be operated by varying the pressure of fluid in a well annulus and applying that pressure to a differential pressure piston within the tool.
The predominant method of creating the differential pressure across the differential pressure piston has been to isolate a volume of fluid within the tool at a fixed reference pressure. Such a fixed reference pressure has been provided in any number of ways.
Additionally, these prior art tools have often provided a volume of fluid, either liquid or gas, through which this reference pressure is transmitted. Sometimes this volume of fluid provides a compressible fluid spring which initially stores energy when the differential area piston compresses that fluid, and which then aids in returning the differential area piston to its initial position.
One manner of providing a fixed reference pressure is by providing an essentially empty sealed chamber on the low pressure side of the power piston, which chamber is merely filled with air at the ambient pressure at which the tool was assembled. Such a device is shown, for example, in U.S. Pat. No. 4,076,077 to Nix et al. with regard to its sealed chamber 42. This type of device does not balance hydrostatic annulus pressure across the power piston as the tool is run into the well. This device does not provide a fluid spring to aid in return of the power piston.
Another approach has been to provide a chamber on the low pressure side of the piston, and fill that chamber with a charge of inert gas such as nitrogen. Then, when the annulus pressure overcomes the gas pressure, the power piston is moved by that pressure differential, and the gas compresses to allow the movement of the power piston. Such a device is shown, for example, in U.S. Pat. No. 3,664,415 to Wray et al. with regard to its nitrogen cavity 44. This type of device does not balance hydrostatic annulus pressure across the power piston as the tool is run into the well. The Wray et al. device utilizes the compressed nitrogen gas in cavity 44 to bias the piston 42 thereof downwardly.
Another approach has been to use a charge of inert gas as described above, in combination with a supplementing means for supplementing the gas pressure with the hydrostatic pressure of the fluid in the annulus contained between the well bore and the test string, as the test string is lowered into the well. Such a device is shown, for example, in U.S. Pat. No. 3,856,085 to Holden et al. When a tool of this type has been lowered to the desired position in the well, the inert gas pressure is supplemented by the amount of the hydrostatic pressure in the well at that depth. Then, an isolation valve is closed which then traps in the tool a volume of well annulus fluid at a pressure substantially equal to the hydrostatic pressure in the well annulus at that depth. Once the isolation valve has closed, the reference pressure provided by the inert gas is no longer effected by further increases in well annulus pressure. Then, well annulus pressure may be increased to create a pressure differential across the power piston to actuate the tool. The Holden et al. device utilizes the energy stored in compression of the nitrogen gas within chamber 128 to assist in returning the power piston 124 to its upper position.
Also, rather than utilize a compressible inert gas such as nitrogen within such tools, it has been proposed to use a large volume of a somewhat compressible liquid such as silicone oil as a compressible fluid spring on the low pressure side of the tool. Such a device is seen, for example, in U.S. Pat. No. 4,109,724 to Barrington.
One recent device which has not relied upon either a large volume of compressible liquid or a volume of compressible gas is shown in U.S. Pat. No. 4,341,266 to Craig. This is a trapped reference pressure device which uses a system of floating pistons and a differential pressure valve to accomplish actuation of the tool. The reference pressure is trapped by a valve which shuts upon the initial pressurizing up of the well annulus after the packer is set. The Craig tool does balance hydrostatic pressure across its various differential pressure components as it is run into the well. The power piston 35 of the Craig device is returned to its original position by a mechanical coil compression spring 36 without the aid of any compressed volume of fluid.
Another relatively recent development is shown in U.S. Pat. No. 4,113,012 to Evans et al. This device utilizes fluid flow restrictors 119 and 121 to create a time delay in any communication of changes in well annulus pressure to the lower side of its power piston. During this time delay, the power piston moves from a first position to a second position. The particular tool disclosed by Evans et al. utilizes a compressed nitrogen gas chamber in combination with a floating shoe which transmits the pressure from the compressed nitrogen gas to a relatively noncompressible liquid filled chamber. This liquid filled chamber is communicated with the well annulus through pressurizing and depressurizing passages, each of which includes one of the fluid flow restrictors plus a back pressure check valve. Hydrostatic pressure is balanced across the power piston as the tool is run into the well, except for the relatively small differential created by the back pressure check valve in the pressurizing passage.
It is apparent from the numerous examples set forth above that it is well known in the prior art to create a trapped reference pressure within a tool by communicating a chamber within the tool with the well annulus, and then isolating that chamber to trap the reference pressure within the tool. In combination with that concept, a number of these prior tools have also utilized a volume of compressible gas or of a relatively compressible liquid such as silicone oil contained within the tool to act as a fluid spring to aid in returning the power piston to its initial position. This compressed gas or silicone oil generally is separated from the trapped well fluid providing the reference pressure by a floating piston so that the trapped well fluid and the compressed gas or silicone oil are always at the same pressure.
Those ones of the various prior art devices discussed above which do utilize a compressible fluid spring to aid in returning the power piston to its original position rely upon the compressibility of the compressed gas or silicone oil, and not upon compressibility of the well fluid itself which may be trapped within the tool.
There are disadvantages inherent in using either a large volume of a relatively compressible liquid such as silicone oil or a volume of compressible gas to account for the volume change within a tool on the low pressure side of the power piston.
When utilizing a tool which provides a sufficient volume of compressible silicone oil to accommodate the volume change required on the low pressure side of the tool, the tool generally becomes very large because of the large volume of silicone oil required in view of the relatively low compressibility thereof.
On the other hand, there is a danger in tools that utilize inert gas such as nitrogen, as there is in any high pressure vessel.